AMD K6-2 400MHz


Faster to come???
k6_2chip.gif (11286 bytes)   Apparently the last of the K6-2 model microprocessors, AMD's release of 366MHz, 380MHz and 400MHz CPUs has been met with both excitement as well as casual disinterest. News of impending release of the K6-3, and the early availability of socket 370 Celerons have created a standoffish attitude in a large percentage of the upgrade market. We all seem to be standing around waiting to see which platform next aligns itself as the guide-on bearer of the high speed low-cost processor market.

These final K6-2s like their predecessors, boast support for AMD's 3DNow! instruction set designed to improve 3D graphics and multimedia performance, and a full 64KB of L1 cache. Availing myself of the purchase of some new processors I recently obtained both 380 & 400MHz parts and while there are some 380MHz parts available with the original core, both of the parts had the code number "26351" impressed directly onto the ceramic casing of the CPU indicating that both parts had new XT cores.

These K6-2s new higher frequency models or steppings as they are referred to in the semiconductor industry, are labeled as as Model 8/[F:8] in documentation to indicate the use of AMD's new XT core, while the original K6-2 core is documented as Model 8/[7:0]. 

Beating The Clock
According to AMD's specifications, the K6-2 XT core translates the original core's 2.0x clock multiplier into a 6.0x clock multiplier defined in the table below, indicating that the original core supports clock multipliers ranging from 2.0x - 5.5x, while the XT core supports clock multipliers ranging from 2.5x - 6.0x, with the 2.0x setting corresponding to a 6.0x clock multiplier.

State of BF[2:0] Inputs

Processor-Clock to Bus-Clock Ratio

100b 2.5x
101b 3.0x
110b 2.0x or 6.0x
111b 3.5x
000b 4.5x
001b 5.0x
010b 4.0x
011b 5.5x
   The ratio selected is dependent on the stepping of the Model 8. The 2.0x ratio is supported on the Model 8/[7:0] (original core design), whereas the 6.0x ratio is supported on the Model 8/[F:8](XT core).

This expresses the the processor's ability to perform at it's rated frequency even without higher speed frontside bus support (i.e. 66MHz x 6.0 = 400MHz).

The XT core architecture has also made some interesting changes in the Write Handling Control Register, providing changes in the Write Merge Buffer and a wider Write Allocate Limiter that allows the processor to cache a larger memory write that isn't already located within the L1 data cache.

Using a burst read to prefetch data located outside the L1 cache, a "Write Allocate" stores the data from the memory write in the processor's L1 data cache. The performance of a single write allocate function is no greater than if the data were written to memory unless the burst read takes place, the real performance edge of write allocate appears in writes to the write allocated cache line following the initial write allocate. This is because the chances that a memory write will occur closer to a previous write are greater than a memory write occurring in an area distal to the previous write.

The performance enhancement comes from using multiple write allocates as opposed to passing the burst read cycles and allowing the memory writes to complete without write allocate intervention, working in tandem with the Write Merge Buffer. Instead of simply leaving non-cacheable write cycles alone, the Write Merge Buffer combines the all the data segments from a given set of memory writes into an 8-byte buffer. By combining all of the writes into the Write Merge Buffer, you reduce processor bus utilization and processor stalls, thus the processes in tandem create an overall increase in performance providing from 6% up to 10% faster execution of integer operations. This increase in performance is most notable in the execution of business-type applications and provides only minimal enhancement for 3D and multimedia applications.

Turn Up The Air Conditioner
The 380 & 400MHz K6-2s differ in their tolerance for operating case temperatures under maximum thermal power.  Where the AMD-K6-2/380AFR CPUs allow for an ambient case temperature in the 0C. 70C range, the AMD-K6-2/400AFQ part requires an ambient case temperature 0C. 60C in range.  This is indicated by the Q in the part number's suffix.

Since the K6-2 400MHz CPU's release back in Nov. many of the super7 mainboard manufacturers have released BIOS upgrades that will allow you to take advantage of the processor's enhanced write allocate features. You'll need to upgrade even if your BIOS appears to correctly identify the CPU on your boot screen. While the processor will work without the BIOS update, your performance can suffer - falling to a level slightly lower than the capabilities of the K6-2 350MHz.

- Performance -

When you get right down to it, with the high frequencies of today's processors, the 20MHz difference in the processors is truly negligible. If you don't have an interest in overclocking that is. Testing both chips on EFA Corp.'s new Viking mS3, a microATX mainboard with onboard sound, and using undocumented settings, it was child's play to push the K6-2 380 up to 400MHz by increasing the FSB from 95MHz to 100 MHz, however,  the chip would not even boot when attempting to increase the clock frequency to 4.5 at 95-100MHz. While it would post and boot at 4.0x112mHz it was wholly unreliable at that speed generating numerous GPFs until it threw a bsod and refused to get more stable even with a slight voltage increase.   The K6-2 400MHz processor overclocks about as well as I expected it to, easily reaching the 450MHz frequency on a 100MHs FSB and increasing the FSB to 112MHz, performed well at 4.0 clock cycles. While it would post at 504MHz the machine refused to boot with the higher external frequency at 4.5 clocks.  You'll need to take extra care with the K6-2 400 as it is more susceptible to heat. 

Test Environment for AMD K6-2 380/400MHz Processors

Mainboard EFA Viking mS3
BIOS- E5SV3-MATX ver1.00/1.01
L2 Cache 1024K
Memory 128MB PC 100 CL2
Viking PE16642U4SN3-2226
Hard Drive Western Digital Caviar AC 36400
Graphics Adapter Matrox Millennium G200 AGP
Operating System Windows 98

Test scores reflect an average of three passes at each speed.  While the mainboard I tested on is no speed demon, it is quite stable and you can expect your performance to increase on par with those described here with an updated BIOS and XT core.

At present the price/performance ratio for both of these processors is quite outstanding with the K6-2 400 in the $160.00 range and the 380MHz model around $20-$25.00 less.  At these prices you can afford to upgrade more than just the processor - increasing system RAM or even a new high-performance graphics adapter. 

While I wouldn't recommend the upgrade for present owners of K6-2 333/350MHz models, the increase in performance is significant enough to warrant the upgrade for K6-2 300MHz owners.  If you are a power gamer, you may however deem it wise to wait for the "Sharptooth" (K6-3), which is most likely to be released in Feb. but at +$100.00 the cost.  Otherwise, both processors are low-cost high performance components that will meet even the most stringent of requirements for the average user.

1998/99 2000 MediaTek